A wrestling ring is the ring stage that professional wrestlers wrestle in.
Wrestling rings are generally composed of an elevated steel beam and wood plank stage covered by foam padding and a canvas mat, with the sides then covered with an "apron" to prevent spectators from seeing underneath. Around the "ring" are three cables (the "ring ropes") wrapped in various types of piping (generally rubber hosing and tape), which are held up by turnbuckles. World Wrestling Entertainment and some independent groups use real rope, as opposed to cables.
Wrestling rings vary in shape and size, with most measuring between 14 and 20 feet on each side, measured between the turnbuckles. WWE uses a 20-foot ring while the past promotions of World Championship Wrestling and Extreme Championship Wrestling used an 18-foot ring. Because of this, the 18-foot ring is commonly held as the "standard" for wrestling rings in the United States and Canada. Rings typically include an "apron" area of the ring floor, extending between one and two feet beyond the ropes; the ring floor is generally between three and four feet above the ground.
Many rings utilize a suspension system with a large coil spring underneath the stage to reduce the impact of a wrestler landing on it; the stiffness of such springs varies, with softer springs providing a gentler impact, at the expense of the wrestlers bouncing visibly on impact; a stiffer spring provides a more "realistic" visual experience, but at a higher risk of injury due to the harsher impact. According to Mick Foley, rings built for the World Wrestling Federation before approximately 1998 were particularly "stiff," and one of them contributed to his injuries suffered during his famous Hell in the Cell match against The Undertaker. A newer style of ring construction utilizes a "flexi-beam" system instead of a spring, where the steel beams used to construct the ring stage absorb much of the impact.
The "traditional" ring -- such as that used by WWE -- is four-sided, but other configurations exist, such as the six sided ring of Asistencia Asesoría y Administración and Total Nonstop Action Wrestling.
The configuration and construction of the "traditional" ring is very similar to (and likely derived from) that of a boxing ring, though the wrestling version has fewer ring ropes (as the standard boxing ring evolved into a four-rope structure), and the ring ropes are not tethered together at their midpoint. Most (if not all) wrestling rings also incorporate more in the way of padding and shock absorbing construction than boxing rings, although this varies according to the preferences of the promoter.
It is a term that originated in traditional Greco-Roman wrestling, since the action takes place on a square mat with a circle painted on it. This format is still used by amateur wrestling leagues throughout the world. Wrestling promoters could've adopted the term from these earlier roots, or it could simply be that it is referred to as a "ring," (circle) but is square in shape.
There is also a belief that it could come from the mathematical problem of squaring the circle.